Conventionally, ink-jet inks are used in which a colorant has been either dissolved or dispersed within a liquid medium containing water as the primary component. With these inks, if the colorant is a dye, then the water resistance and lightfastness of the image following printing is poor, and the ink cannot be used in outdoor applications or the like.
On the other hand, ink compositions in which the colorant is a pigment with excellent lightfastness are widely used for black inks that use carbon black.
However, in order to enable the use of aqueous ink compositions containing dispersed organic pigments, and particularly chromatic colored organic pigments, in the formation of multicolored images, a favorable color strength for the ink-jet ink, and a hue that can be combined with other colored inks to form an image with favorable color reproducibility are both important factors, and a variety of pigments which have different hue are being investigated for use within ink-jet recording.
A multitude of pigments are being investigated for yellow ink-jet inks, including pigment yellow 3, 12, 13, 14, 17, 24, 34, 35, 37, 42, 53, 55, 74, 81, 83, 95, 97, 98, 100, 101, 104, 108, 109, 110,117, 120, 138, 147, 151,153, 180, and 181. However, as for these pogments, not many can necessarily be used favorably for ink-jet inks.
In order for aqueous ink-jet ink compositions with these yellow organic pigments dispersed therein to be able to offer at least an equivalent level of practicality to dye inks, while also taking advantage of the characteristics provided by the pigments, the pigments must be grinded very finely so that no coarse particles capable of impeding the ink-jet ejecting exist in the ink, and the inks need to provide similar long-term ejecting stability and storage stability to that provided by dye-type inks. However in many cases, yellow pigments that exhibit excellent hue and color strength, and suffer no particular problems within normal applications suffer from inadequate dispersibility, storage stability, and/or discharge stability if used within ink-jet inks, meaning they cannot be used for ink-jet applications.
Furthermore, because many yellow pigments are prone to problems of lightfastness, an ideal pigment cannot be necessarily selected solely on the basis of the hue immediately after image formation. Moreover, depending on the pigment, crystal growth during synthesis can increase the size of the primary particles, causing a deterioration in the transparency of the coloring, or a worsening of the ink ejecting properties. Therefore, depending on the pigment used, the addition of a dispersion assistant to the ink composition may also need to be considered (see patent reference 1: Japanese Unexamined Patent Application, First Publication No. Hei 10-158555, patent reference 2: Japanese Unexamined Patent Application, First Publication No. 2001-98200, patent reference 3: Japanese Unexamined Patent Application, First Publication No. Hei 7-126546, and patent reference 4: Japanese Unexamined Patent Application, First Publication No. 2003-253188).
Accordingly, in a yellow ink-jet ink composition, selecting a yellow pigment that totally satisfies the required levels of color strength, hue, lightfastness, and storage stability and the like is extremely difficult, and conventionally, a pigment that provides a favorable balance across the above characteristics is selected, and any drawbacks associated with the pigment are covered by adjusting the ink blend or the production method to enable the composition to be used as an ink-jet ink.
For example, an improvement in the lightfastness of C.I. Pigment Yellow 74 by adding a styrene-(meth)acrylic acid-based water-soluble resin has been reported (see patent reference 5: Japanese Unexamined Patent Application, First Publication No. Hei 11-199811).
For example, by using C.I. Pigment Yellow 128 in combination with Pigment Yellow 110, favorable wide color reproducibility across a wide range is achieved (see patent reference 6: Japanese Unexamined Patent Application, First Publication No. 2001-81369).
For example, by using C.I. Pigment Yellow 93 in combination with a styrene-acrylic acid-based copolymer and an acetylene glycol compound, the ejecting properties and storage stability are improved (see patent reference 7: Japanese Unexamined Patent Application, First Publication No. 2002-226738).
As for the above characteristics, dispersibility and storage stability are particularly important, as they directly affect the ink ejecting, and favorable storage stability at high pigment concentrations is necessary for ensuring favorable color reproducibility.
Accordingly, a variety of dispersants and dispersion methods are being investigated to improve the dispersibility and storage stability of aqueous pigment dispersions.
For example, a polymer of acrylic acid, styrene, and a-methylstyrene has been used to improve the ejecting stability at the nozzle and the water-resistance of the printed image for an ink-jet ink containing an azo-system organic pigment (see patent reference 8: Japanese Unexamined Patent Application, First Publication No. Hei 8-253716).
For example, the content of 0.01 to 5% by weight of either a polymer containing carboxyl groups, with an acid number of 50 to 700, or a polymer containing carboxylate salt groups has been disclosed, and styrene-maleic anhydride, a hydrolysis product of styrene-maleic anhydride, or a salt or ester thereof is used as the polymer (see patent reference 9: Japanese Unexamined Patent Application, First Publication No. Hei 9-157564).
For example, a solid kneaded material can be prepared by conducting high-viscosity kneading using a styrene-acrylic resin together with a pigment, a basic compound, and a wetting agent, thereby yielding an aqueous pigment dispersion with favorable storage stability that can be used as an ink composition for ink-jet recording, wherein a resin in which the monomer composition ratio is styrene/methacrylic acid/acrylic acid=77/13/10 (weight ratio), and the weight average molecular weight is within a range from 3,000 to 50,000 can be used as the styrene-acrylic resin (see patent reference 10: Japanese Unexamined Patent Application, First Publication No. 2003-226832).
In this manner, in order to take advantage of the color strength and water resistance of pigments, while also achieving levels of storage stability and ejecting stability similar to those of dye inks, it is important that, firstly, a yellow pigment with a favorable balance of characteristics is selected, and this pigment must then be combined with the most appropriate resin for that pigment in order to impart superior levels of dispersibility and storage stability.
However, many of the references relating to resin dispersants, including those listed above, disclose only general comment relating to resins as dispersants for use within ink-jet inks or aqueous dispersions, and no mention is made of selection of the most appropriate yellow pigment when any of the various resin dispersants is combined with a yellow pigment.
By selecting the most appropriate combination of yellow pigment and resin dispersant, and then optimizing factors such as the composition, molecular weight and blending of the resin dispersant, a yellow aqueous pigment dispersion with excellent levels of storage stability and dispersibility can be prepared. However, this type of detailed investigation has yet to be conducted for all of the various yellow pigments, and well-balanced aqueous pigment dispersions which provide excellent color strength, lightfastness, and color reproducibility, while also offering favorable dispersibility and storage stability are keenly sought.
Particularly in the case of aqueous pigment dispersions available to ink-jet inks for thermal jet systems, excellent storage stability during high-temperature storage is also preferred, but a yellow ink-jet ink composition that provides excellent color strength, lightfastness, and color reproducibility, while also exhibiting this type of superior storage stability has yet to be reported.
An object of the present invention is to provide a well-balanced aqueous pigment dispersion that uses a yellow pigment, which has favorable dispersibility and storage stability, excellent color strength, lightfastness, water resistance and ejecting stability, and can be used for preparing an ink-jet ink. In addition, an object of the present invention is to provide an aqueous pigment dispersion which in addition to the performance factors described above, also exhibits favorable storage properties at high temperature, and can be used for preparing a thermal jet-type ink-jet ink with favorable discharge stability.